immediateearly

Immediate early genes (IEGs) are a class of genes that are rapidly and transiently activated in response to cellular stimuli, often without the need for new protein synthesis. In neurons and many other cell types, IEGs are among the first genes expressed after activation and frequently encode transcription factors or regulatory proteins that shape subsequent gene expression programs. Their expression is typically detectable within minutes of stimulation and returns to baseline within a few hours.

Activation of IEGs is driven by intracellular signaling pathways triggered by stimuli such as calcium influx, growth factors, or neurotransmitter activity. Key pathways include MAPK/ERK, CaMK, and CREB-dependent transcription. The rapid response is facilitated by pre-existing transcriptional machinery and promoter regions poised for transcription. Common examples of IEGs are c-fos, EGR1 (also known as NGFI-A), JUN family members, and Arc (Arg3.1). The exact set of induced IEGs varies by cell type and stimulus.

Functionally, IEGs often encode transcription factors or signaling components that regulate a second wave of gene expression, enabling cellular adaptations such as synaptic plasticity in neurons. Because their rapid induction mirrors recent activity, IEGs are widely used as markers of cellular activation to map active circuits and study activity-dependent changes.

In summary, immediate early genes represent a rapid, transient transcriptional response to stimuli, bridging initial signaling events and longer-term cellular programs. The term is used across contexts to describe promoters and genes that respond quickly without requiring new protein synthesis.